JIRA: https://jira.spring.io/browse/INT-3572
Previously, a blocking queue was used to hold seen files; this was used to enable
FIFO when a max capacity is set.
When used with no capacity, a queue is not needed; also the `contains` operation
on a queue requires a linear search which does not scale well for a large number
of files.
Use a `HashSet` instead to significantly improve the `contains` performance.
Only maintain a queue if a max capacity is set.
Fix `AcceptOnceFileListFilterTests` to be compatible with Java < 8
26dae7bae7